1. Field of the Invention
This invention relates to a composition suitable for the forming of metal oxide coatings on refractory substrates. More particularly, this invention relates to a nonflammable, high-boiling composition which is suitable for producing a colored, substantially transparent metal oxide coating on a freshly formed float glass.
2. Description of the Prior Art
Transition metal oxide coatings on refractory substrates such as glass are well known in the art. The coatings are colored and are useful as solar radiation filters in architectural glass applications. The metal oxide coatings can be formed by applying in an oxidizing atmosphere a solution of an organo-metallic compound dissolved in an organic vehicle to a refractory substrate which has been preheated to a temperature of about 900.degree.-1200.degree. F. On contacting the heated substrate, the solution is believed to pyrolyze, and a metal oxide coating is formed on the surface of the substrate. Suitable organo-metallic compounds for the above-described process are 2-ethyl hexonate salts of transition metals. These compounds and their use in forming metal oxide coatings by pyrolytic techniques is described in U.S. Pat. No. 3,185,586. Other types of organo-metallic compounds which can be used for forming metal oxide films are the transition metal 1,3-beta diketonates, such as are described in U.S. Pat. No. 3,202,054. The transition metal beta diketonates are particularly interesting because they have the ability to form a metal oxide coating of extremely uniform thickness over a large area of substrate. Unfortunately, the beta diketonates of transition metals are only slightly soluble in many of the commonly employed organic solvents. For example, as disclosed in U.S. Pat. No. 3,202,054, transition metal 1,3-beta diketonates are soluble in such organic vehicles as methanol, methanol-pyridine mixtures and methanol-benzene mixtures. Using such solvents in a pyrolytic-type process, particularly on a float line, is dangerous because many commonly used solvents and their vapors are toxic, very flammable and are relatively low-boiling.
In coating float glass, it has been found, as disclosed in U.S. Patent Application Ser. No. 16,670, filed Mar. 5, 1970, that it is best to apply a coating composition to the glass just after it has been freshly formed. When the coating composition is applied at this point, the temperature of the float glass environment is about 1100.degree. F. Using a coating composition which contains a transition metal beta diketonate dissolved in any of the above-described prior-art solvent systems presents a definite toxicity problem and a fire hazard. Also, since the above solvents are relatively low-boiling, they evaporate or pyrolyze before the solution contacts the glass surface. As a result, particles of solute, rather than a true solution, hit the hot glass surface which results in a metal oxide coating having a very poor texture.